The present invention relates to an air-fuel ratio control system for an internal combustion engine.
There is well known an air-fuel ratio control system wherein the air-fuel mixing ratio of a combustible mixture to be fed to the engine is controlled to a value approximating to or equal to a stoichiometric value on a feedback scheme in dependence on the concentration of a constituent of the exhaust gases emitted from the engine. In practice, the feedback scheme is enabled only during any one of the medium and low engine operating conditions to render a composition signal, representative of the concentration of the exhaust gas constituent, to be actually used in controlling the air-fuel mixing ratio to the stoichiometric value for the purpose of minimizing the emission of obnoxious components of the exhaust gases.
However, during the high load engine operating condition such as, for example, the acceleration of the engine, the feedback scheme referred to above is disabled to permit the system to control the air-fuel mixing ratio to a lower value than the stoichiometric value. This is because, if the combustible mixture is controlled to have an air-fuel mixing ratio approximating to or equal to the stoichiometric value even during the high load engine operating condition in which a relatively high engine power output is required, the engine will fail to give the high engine power output.
With this type of control system, it has been found that the repeated occurrence of the high load engine operating condition such as often experienced during the city cruising in which acceleration is repeated results in the enriched combustible mixture and, hence, the emission of a relatively great amount of the exhaust gas components to the atmosphere.
The Japanese Laid-open patent publication No. 53-8427, published Jan. 25, 1978, discloses an air-fuel control system which appears to be effective to substantially eliminate the above described disadvantage. According to this publication, the air-fuel control system is so designed as to disable the feedback scheme in the event that air is sucked into the engine at a rate higher than a predetermined value for a length of time greater than a predetermined value. In other words, the control system according to the above mentioned publication, is so designed that, only when and after the engine operating condition in which the relatively high engine power output is required has lasted for a length of time greater than the predetermined value, the combustible mixture to be supplied to the engine is enriched.
However, with the prior art control system such as disclosed in the above mentioned publication, it has been found disadvantageous in that the required engine power output can not be obtained at the time of start of the acceleration with an accelerator pedal fully depressed and, therefore, the acceleration feeling will be adversely affected.